THALLOPHYTA. 623conditions remain favourable for further growth. When the substratum isexhausted, &c., spores are formed which can remain for long periods—until, indeed,circumstances are again favourable for renewed development. These spores mayarise either inside the bacterial cells (= endospores), the protoplasm contractingsomewhat and forming a new wall around itself, or the cells may become trans¬formed entirely into spores ( = arthospores), the wall becoming specially thickened.In no case is there any sexual process. Whilst many Bacteria are only knownunder certain forms and are regarded as species of the genera Micrococcus, Bac¬terium, Bacillus, Spirillum, &c., others are known which, in the course of theirdevelopment, pass through several such forms, and are termed pleomorphic. Thatall Bacteria are thus pleomorphic seems improbable, though the discovery thatpleomorphism existed at one time led to the wildest generalizations. Largenumbers of Bacteria display an active movement which, though formerly attri¬buted to various contractions of their bodies, are now kno"wn to be due to cilia.These cilia may be borne in tufts of 5 or 6 at the two ends of the organism, as inSpirillum Undula, or they may be solitary at one extremity, as in the CholeraBacterium (Spirochaete choleroe asiaticce), or they may be scattered over the surfaceof the organism, as in the Hay-bacillus, Bacillus subtilis. It is due to the extremefineness of these cilia that they were not recognized long ago. Though the forms under which Bacteria occur are relatively few, their mode oflife and special activity is exceedingly varied. The interest attaching to Bacteriarests largely on their effects on the substratum from which they draw their food.Taking, first, the saprophytes. These split up their substratum into simple sub¬stances. In some cases there is a complete oxidation, with production of carbondioxide and water; in others this is only partial, as in some of the cases offermentation, e.g. when alcohol is oxidized into acetic acid by the activity of thevinegar organisms Bacillus and Micrococcus aceti (cf. figs. 368^ and 368*). Orthere may be a decomposition unaccompanied by simple oxidation, as in manycases of fermentation, e.g. as when sugar is split into alcohol and carbon dioxide.Often these operations are accompanied by the development of a foul-smelling gas,when we speak of putrefaction. The number of saprophytic Bacteria which excitecharacteristic splittings in their substrata is considerable. In addition to thosealready quotfed, we may mention Bacillus Amylohacter, the organism of butyricacid fermentation; Bacillus lacticus, which causes milk to become sour; Leuconostocmesenterioides, which has the power of converting large quantities of sugar into agelatinous mass in a very short space of time. Again, in a number of forms theproduction of a special colouring matter is associated with the activity of theorganisms, as is the case with Micrococcus prodigiosus (cf. fig. 368 ^), the "blood-portent" which makes its appearance on various starchy food-stuffs, and Beggiatoaroseo-persicina, found on decaying vegetable matter in water, and known as"peach-mud". Many Bacteria are parasitic in the bodies of animals, and someamong them are harmless. This is the case with Sarcina ventricuU (fig. 368^"),known only in the human alimentary canal in the form of packets of cells. Harm-